1. Field of the Invention
This invention relates to an imaging equipment employing ultrasonic energy, and more particularly to an acoustic microscope.
2. Description of the Prior Art
In recent years, it has become possible to generate an ultrasonic wave having as high a frequency as 1 GHz and hence to realize an ultrasonic wavelength of approximately 1 .mu.m in the water. As a result, an acoustic imaging equipment of high resolving power has been fabricated. In the equipment, a collimated acoustic beam is formed by the use of a concave lens, and a high resolution attaining to 1 .mu.m is realized.
A specimen is inserted in the beam, and an acoustic wave reflected by the specimen is detected to obtain information expressive of the elastic properties of the specimen, or the specimen is mechanically scanned to prepare an image.
Problems in the case of obtaining the ultrasonic image in this way will be described with reference to FIGS. 1 and 2.
FIG. 1 is a diagram which shows the schematic construction of a transducer for obtaining a reflected signal from a specimen. In the figure, an acoustic propagating medium (a cylindrical crystal of, for example, sapphire or silica glass) 20 has one end face optically polished into a flat surface and the other end face formed with a semispherical hole 30. A piezoelectric thin film 10 is deposited on the flat surface of the crystal 20. An RF pulse acoustic wave which is a plane wave is radiated into the crystal 20 by an RF pulse electric signal applied to the piezoelectric thin film 10. The plane acoustic wave is collimated onto a specimen 50 situated at a predetermined focus, by a positive lens which is formed of the interface between the spherical hole 30 and a medium (in general, water) 40 (accordingly, the spherical hole becomes the aperture of the lens). An acoustic wave reflected by the specimen 50 is collected and converted into a plane wave by the same lens, and the plane wave is propagated through the crystal 20 and is finally converted into an electric signal by the piezoelectric thin film 10. These circumstances are observed in a video frequency range as illustrated in FIG. 2. Here, the axis of abscissas represents the time, and the axis of ordinates the intensities of signals. Letter A indicates a transmitted echo, letter B an echo from the lens boundary 30, and letter C a reflected echo from the specimen 50. The reflected echo C varies depending on the acoustic properties of the specimen or through the scanning of the specimen, whereas the echoes A and B are inherent to the transducer and are constant. In detecting the reflected echo C, the presence of the echoes A and B incurs the following disadvantages:
1 It becomes an obstacle in case of performing the time gating.
2 The saturation of a receiver amplifier takes place, and the recovery time of the amplifier attendant thereupon is caused.
In this manner, the echoes A and B other than the reflected echo C from the specimen act as unwanted or parasitic peak signals in the case of obtaining the ultrasonic image.
Heretofore, in order to eliminate the unwanted or parasitic peak signals, the time gating has been relied on. However, when the aperture of the lens is small, the echoes B and C come close to each other and are very difficult to discriminate.
The reason is that, in order to enhance the resolving power of the imaging system, the frequency needs to be raised, which however gives rise to a great decay of the acoustic wave in the propagating fluid.